Bicomponent filaments of the sheath/core configuration are well known and a variety of spinning packs and spinnerets have been employed in the production of such filaments. A conventional spinning assembly involves feeding the sheath-forming material to the spinneret orifices in a direction perpendicular to the orifices, and injecting the core-forming material into the sheath-forming material as it flows into the spinneret orifices.
A bicomponent spinning assembly is disclosed in U.S. Pat. No. 4,406,850 whereby molten sheath polymer is issued in ribbon flow into recessed slot-like portions of the top surface of the spinneret positioned between rows of raised spinneret core inlets. U.S. Pat. No. 4,251,200 also discloses a bicomponent spinning assembly comprising a spinneret plate and a distribution plate spaced apart, the distributor plate having an aperture opposite each orifice in the spinneret plate and a plateau-like protrusion extending about the axis common to aperture and the extrusion orifice. Additionally, the assembly includes an orifice plate for restricting the entrance to the orifice.
The concentricity of the core and sheath capillaries in the prior art spinning assemblies as described above and in other spinning assemblies is not satisfactory. It is difficult to properly position the distributor plate and the spinneret of the prior art assemblies so that proper alignment of the distributor and flow passages and pressure drop control are obtained so as to produce sheath/core bicomponent fibers of uniform cross section.
Typical of spinning assemblies of the prior art as exemplified by the cited references, the gap between the exit surface of the distributor and the inlet surface of the spinneret is fixed. Thus, if the sheath polymer viscosity varies or the core sheath ratio changes, the pressure drop control in the prior art assemblies is lost. It is necessary to control sheath polymer pressure drop adjacent the spinneret inlet as will be hereafter discussed to obtain bicomponent fibers consistent from filament to filament.
Further, in those spinning assemblies where the annular gap between the distributor and spinneret is fixed, polymer pressure is sufficient at times to bow the spinneret away from the distributor thereby opening up the gap and changing the pressure drop. The exit and inlet passages of the distributor and spinneret, respectively, nearest the center and the source of the sheath polymer will have the widest gaps and those farthest from the center will have the narrowest gap. Sheath polymer will flow preferentially to the inner passages providing poor bicomponent filament uniformity.